1. Field of the Invention
The present invention generally relates to grid-connected electrical power systems and, in particular, to systems and methods for preventing islanding of such systems.
2. Description of the Related Art
Recently there has been a resurgence of concern about islanding of grid-connected electrical power systems. Such electrical power systems include photovoltaic (PV) and wind-powered systems, among others. Islanding occurs when such a system continues to energize a section of the grid after that section has been isolated from the main utility voltage source. Generally, islanding is undesirable because it poses a safety hazard to utility service personnel, and also because it can lead to asynchronous reclosure which can damage equipment. It is therefore important that electrical power systems incorporate methods to prevent islanding.
Consider an electrical power system, i.e., a PV system, connected to a feeder line which is in turn connected to the utility grid through a transformer and some sort of switch (a recloser, breaker, fuse, etc.). The PV system consists of a PV array and a power conditioning unit (PCU). A local load also is connected to the feeder line. If the switch were opened, under certain conditions, it is possible for the PV PCU to continue to energize the isolated section of the grid and supply power to the local load. This is xe2x80x9cislanding,xe2x80x9d and the isolated section of the utility being powered by the PV system is referred to as an xe2x80x9cisland of supplyxe2x80x9d or, simply, xe2x80x9can island.xe2x80x9d Utilities frequently use the term xe2x80x9crenewable energy islandxe2x80x9d to differentiate this situation from other types of islands. Although this distinction is frequently important, for brevity, we will use the term xe2x80x9cislandxe2x80x9d throughout without ambiguity.
The amount of time between the disconnection of the utility and the shutdown of the PCU is referred to as the run-on time. Islanding events typically are subdivided into two categories: long-term, with run-on times of one second or more, and short-term, with run-on times of less than one second.
As mentioned briefly hereinbefore, the primary concern with long-term islanding is one of safety. For example, maintenance or repair personnel arriving to service the isolated feeder may be unaware that it is still energized, which could lead to personal injury. This is of particularly great concern in the case of scheduled maintenance, when the switch would be manually operated by service personnel who typically immediately commence work on the isolated system. In this case, islanding of even a few tenths of seconds could be dangerous.
Another problem associated with both long-term and short-term islanding is that the electrical power system, which relies on the utility voltage to provide a phase and frequency reference for its output current, may lose synchronism with the utility while the switch is open. The utility could then reclose on an electrical power system which is out of phase. Most electrical power PCUs are two quadrant devices, designed for unidirectional power flow from the DC to AC side only. This typically is done for economic reasons; two-quadrant converters are less expensive than four-quadrant converters. However, during an out-of-phase reclosure, there are intervals in which the polarities of the voltage and current are opposite. During these intervals, the converter is absorbing power from both the PV, in the case of a PV system, and utility sides, which can lead to destructive component failures in the PCU.
It has been postulated that another possible problem with short-term islanding is that it can interfere with the arc-clearing function of protective relays. However, there is much debate over whether this is a significant issue.
Another problem, which is increasing in relevance, is that some islanding prevention methods interfere with each other, leading to longer run-on times, and possibly failure to detect islanding if several electrical power systems are present in the island. In some cases, this can happen even if all the electrical power systems in the island are using the same islanding prevention scheme. This situation has been termed the xe2x80x9cmulti-inverter case,xe2x80x9d and it could become increasingly common with the proliferation of grid-connected electrical power systems, such as roof-mounted PV arrays, for instance, and the development of PCUs for AC PV arrays, in which case there could be tens or even hundreds of PCUs in an island.
Therefore, there is a need for systems and methods which address these and other shortcomings of the prior art.
Briefly described, the present invention provides an electrical power system which incorporates a positive feedback feature for preventing islanding. In a preferred embodiment, the electrical power system includes a power conditioning unit which is configured to receive the DC electrical output signal and to deliver an AC output signal to a grid-connected load. Preferably, the power conditioning unit includes a controller which is configured to monitor the AC output signal so that the power conditioning unit may cease delivering the AC output signal when a characteristic of the AC output signal satisfies an established criterion.
In accordance with an aspect of the present invention, some embodiments may incorporate a controller which is configured to accelerate a shift in frequency of the AC output signal via a modification signal so that either an over-frequency relay or an under-frequency relay of the power conditioning unit prevents delivery of the AC output signal from the power conditioning unit.
In accordance with another aspect of the present invention, a power conditioning unit for use in a photovoltaic system is provided. Preferably, the power conditioning unit includes an inverter, a controller, and a feedback loop, with the inverter being configured to receive the DC electrical output signal of a photovoltaic array, to convert the DC electrical output signal into an AC output signal, and to deliver the AC output signal to a grid-connected load. The feedback loop is configured to provide the controller with information regarding the AC output signal, and the controller is configured to deliver a modification signal to the inverter so that the AC output signal changes in response to the modification signal. So configured, the power conditioning unit may cease delivering the AC output signal when a characteristic of the AC output signal satisfies an established criterion.
In accordance with another aspect of the present invention, a computer readable medium is provided for reducing size of a non-detection zone in a grid network, where the grid network includes an electrical power system adapted for providing power to a grid-connected load. Preferably, the computer readable medium includes: a first code segment configured to measure frequency deviation between an output signal of the grid and an output signal of the electrical power system; a second code segment configured to apply the measured frequency deviation to the output signal of the electrical power system; and, a third code segment configured to discontinue the output signal at the power conditioning unit when the measured frequency of the output signal satisfies an established cut-off level.
In accordance with yet another aspect of the present invention, a preferred method for reducing size of a non-detection zone in a grid network, where the grid network includes an electrical power system adapted for providing power to a grid-connected load, is provided. Preferably, the method comprises the steps of: (1) measuring frequency deviation between an output signal of the grid and an output signal of the electrical power system; (2) applying the measured frequency deviation to the output signal of the electrical power system; and (3) when the measured frequency of the output signal of the electrical power system satisfies an established cut-off level, discontinuing the output signal. In some embodiments, the step of applying the measured frequency deviation includes using the measured frequency deviation as an argument of a function F.
Other features and advantages of the present invention will become apparent to one of reasonable skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional objects, features, and advantages be included herein within the scope of the present invention, as defined by the claims.